Examining The Effects Of Ocean Acidification On Marine Ecosystems

December 18, 2008

Concern about increasing ocean acidification has often focused on its potential effects on coral reefs, but broader disruptions of biological processes in the oceans may be more significant, according to Donald Potts, a professor of ecology and evolutionary biology at the University of California, Santa Cruz, and an expert in coral reef ecology and marine biodiversity.

Potts gave an invited talk on “Geobiological Responses to Ocean Acidification” at the Fall Meeting of the American Geophysical Union (AGU) in San Francisco on Wednesday, December 17.

Ocean acidification is one of the side effects of the rising concentration of carbon dioxide in Earth’s atmosphere due to the burning of fossil fuels. The oceans can absorb enormous amounts of carbon dioxide from the atmosphere, but as the gas dissolves it makes the water more acidic. Increasing acidity can make life difficult for corals and other marine organisms that build shells and skeletons out of calcium carbonate.

Scientists fear that acidification will slow the growth of these organisms and cause calcium carbonate structures to dissolve. Potts agrees that dissolving shells will certainly be a problem for many marine organisms, but he thinks the disruptions will run much deeper.

“It’s not just a question of coral reefs, and it’s not just a question of calcification,” he said. “What we are potentially looking at are disruptions of developmental processes and of populations and communities on many scales.”

The term “acidification” refers to a slight lowering of the pH of ocean water, pushing it closer to the acidic end of the scale, although it is still slightly alkaline. A small decrease in pH affects the chemical equilibrium of ocean water, reducing the availability of carbonate ions needed by a wide range of organisms to build and maintain structures of calcium carbonate.

Many phytoplankton–microscopic algae that form the base of the marine food web–build calcium carbonate shells to protect themselves from microscopic predators called ciliate protozoa. A disruption of the ability of phytoplankton to build their shells could have ripple effects throughout the marine food web, Potts said.

“It’s going to change the dominant organism in the food chain, and there’s a very real danger that it may short-circuit the food chains,” he said. In other words, ciliate protozoa gorging on unprotected phytoplankton may flourish at the expense of other organisms higher up the food chain.

But calcification of shells is not the only biological process affected by acidification, Potts added. “All biochemical physiological reactions are potentially going to change,” he said. Developing organisms are most likely to be affected, due to their low range of environmental tolerances, but it is unclear what the ecological ramifications will be.

Ocean acidification may not affect all parts of the oceans equally. Within 100 kilometers (62 miles) of shore, the pH of ocean water is more variable than in the rest of the ocean. Fresh water and wind from the land can carry chemicals that alter the pH of near-shore water, making it either more acidic or more alkaline. There may be organisms in this region that are already starting to adapt to changes in ocean acidity, Potts said.

“We should be thinking in terms of triage,” he said. “We want to be predicting where are the organisms that are most likely to survive or survive the longest, and this is where we should be concentrating our conservation and management efforts, given finite resources.”